Acoustic wave device operating principles are well-known in the prior art. Early theory for wave propagation in multi-layered media was introduced by Lord Rayleigh in the last century and resulted in a derivation for acoustic waves traveling along the free surface of a semi-infinite elastic half-space. This derivation predicted an expression of the velocity of the propagating surface wave. Later, Stoneley described waves traveling along the interface between two different elastic solids. Subsequent studies addressed the conditions when these waves travel without leaking into the solids and formed the leaky wave theory. Lamb added another interface to introduce a flat layer of finite thickness. His derivation predicted symmetric modes and anti-symmetric modes in what has become well-known in the art as Lamb wave dispersion curves.
Lamb waves are commonly used to make measurements, for example, measurements of elastic properties of plate and shell structures. Studies have been conducted to monitor fatigue damage in composite samples using Lamb wave velocity measurements. Other measurements that may be made include monitoring of ultrasonic flexural Lamb waves in thin films for sensing and actuation of devices. In additional applications, the Raleigh--Lamb dispersion relation that governs the propagation of Lamb waves in free, homogenous, isotropic plates is used to derive the dispersion relation for the lowest order flexural (M21) mode in terms of in-plane tension, stiffness, mass per unit area and rotary and inertia.
However, Lamb waves often exist in both lower and higher order modes simultaneously making the interpretation of the wave data more complex and difficult. Recent research includes the use of transducers mounted at the front end of a specimen to increase mode sensitivity by selectively exciting lower order symmetric (M11) and anti-symmetric (M21) Lamb wave modes in thin solid plates. Reference, for example, the research of F. Levent Degertekin and Butrus T. Khuri-Yakubal in a paper entitled "Single Mode Lamb Wave Exitation In Thin Plates By Hertzian Contacts" printed in Applied Physics Letter 69 (2) on Jul. 8, 1996. This research used Hertzian contacts formed between the plates and the end of specially designed quartz rods which guide extensional waves generated by transducers bonded at the end of the rods.
It is also known in the prior art that the use of an amorphous metal as a delay line may provide electrically efficient performance. Reference, for example, a paper written by Webb, Gorester, Ganguly and Vittoria for the Naval Research Laboratory that used a magnetostrictive ribbon for experimentation. Those concerned with the use and development of acoustic wave devices recognize the need in the art for a simple, low-cost, miniature device that can excite pure, lower order Lamb waves.